1. Field of the Invention
The present invention relates to apparatus for controlling a blower motor composed of a brushless motor.
2. Description of the Prior Art
Blowers of automobile air-conditioners generally comprise a sirocco fan driven by a motor. When the operation of the automobile air-conditioners is changed from the vent mode in which the flow resistance within an air-flow duct is relatively small, to the defrost mode in which the flow resistance in the air-flow duct is relatively large, the performance characteristics of the conventional blowers are such that the flow rate of the sirocco fan decreases while at the same time the motor rotational speed increases. At the time of this mode change, if a control is achieved to maintain the speed of rotation of the motor at a constant level, the flow rate of the sirocco fan will be considerably reduced in the defrost mode.
In recent years, a demand for a silent vehicle passenger compartment has greatly increased with the prolonged service life of motor vehicles. This demand involves a study of sound reduction of a blower motor which constitutes a noise generating source. The conventional blower motor is equipped with brushes and hence is structurally limited as to the reduction of the noise level.
With this difficulty in view, an attempt has been made to realize a blower motor composed of a brushless motor as the size and manufacturing cost of the brushless motor are progressively reduced. To control rotation of such a brushless motor, the speed of rotation of a rotor of the brushless motor is detected by a Hall-effect element rotor and a signal proportional to the detected rotor revolution speed is converted into a voltage signal which in turn is compared with a predetermined reference voltage so as to maintain the difference between two compared signals at a constant value. This rotation control system is disclosed in Japanese Patent Laid-open Publication No. 58-36184. Japanese Patent Laid-open Publication No. 58-54884 discloses another rotation control system in which the power voltage is regulated with a change in rotational speed of the brushless motor.